Why do photons have momentum yet no mass?

In summary, the equation e = pc is the correct equation to use when dealing with particles traveling at the speed of light. This equation shows the relationship between a particle's energy and momentum. The equation p = mv is not applicable near the speed of light and is instead replaced by the relativistic equation, which takes into account a particle's mass and velocity. The reason for this is due to the nature of photons and their high energy and frequency.
  • #1
chestycougth
15
0
I imagine this gets asked a lot but I'm looking for an in depth explanation since none of the others I've found are detailed enough.

I understand that p = mv is not applicable near the speed of light and I think (but am unsure) that the correct equation to use is e = pc.

Assuming that e = pc is the correct equation to use then what does 'p' represent?

Assuming e = pc is the wrong equation then what is the right one?

And can someone (if you can be bothered to waste time on a dumbass like me) explain why p = mv is not applicable?

Thanks.
 
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  • #2
No one knows 'why' a photon exists nor why it has the characteristics it does...
the fundamental particles of this universe, as well as mass, energy, time, distance,etc...were all 'unified' [combined] in one entity very early in our universe...that was a very high energy unstable environment which underwent what is called 'spontaneous symmetry breaking'...meaning the original entity broke down to a lower energy state and became all those different entities I listed... which is what we observe today.

with p = mv, what would you use the the 'm' of a photon??

For a correct description, see here:

http://en.wikipedia.org/wiki/Photon

Unlike a matter particle, a photon always moves in a vacuum at 'c' and if a photon has higher energy it has higher frequency...so a gamma ray, for example, is very energetic [high frequency] visible light less so.
 
  • #3
chestycougth said:
I imagine this gets asked a lot but I'm looking for an in depth explanation since none of the others I've found are detailed enough.

I understand that p = mv is not applicable near the speed of light and I think (but am unsure) that the correct equation to use is e = pc.

Assuming that e = pc is the correct equation to use then what does 'p' represent?

Assuming e = pc is the wrong equation then what is the right one?

And can someone (if you can be bothered to waste time on a dumbass like me) explain why p = mv is not applicable?

Thanks.

Please note that even in the classical treatment of light as EM wave and not photons, there is still a momentum associated with light! This is a standard treatment of classical E&M. So one does not need to have a picture of massless photons to already realize that light can have a momentum.

Zz.
 
  • #4
chestycougth said:
II understand that p = mv is not applicable near the speed of light and I think (but am unsure) that the correct equation to use is e = pc.

The relativistic relationship between a particle's energy, momentum and mass is

$$E^2 = (pc)^2 + (mc^2)^2$$

where m is what many people call "rest mass" but physicists usually call just "mass." Setting m = 0 (as for a photon) gives you E = pc.
 
  • #5
I understand that p = mv is not applicable near the speed of light...

It IS applicable NEAR but not AT the speed of light...

edit: sloppy explanation by me...see last line of the next post..by Bill_K..
 
Last edited:
  • #6
I understand that p = mv is not applicable near the speed of light...
It IS applicable NEAR but not AT the speed of light...
Only if you mean m to be the antiquated concept of relativistic mass. In terms of the usual rest mass, p = γmv.
 

1. Why do photons have momentum if they have no mass?

This is due to the fact that photons, as particles of light, have both energy and momentum, which are related by the equation E=pc. While photons have no rest mass, they do have energy and therefore have momentum.

2. How can something with no mass have momentum?

Momentum is not solely determined by mass, but also by velocity. Since photons are always moving at the speed of light, they have a non-zero momentum despite having no rest mass.

3. What is the significance of photons having momentum?

The momentum of photons plays a crucial role in many physical phenomena, such as the photoelectric effect and the Compton effect. It also helps explain the behavior of light in terms of wave-particle duality.

4. Can photons be affected by forces if they have no mass?

Yes, photons can experience forces such as gravity and electric fields, even though they have no mass. This is because they have energy and momentum, which can be affected by these forces.

5. How does the momentum of photons compare to that of massive particles?

The momentum of photons is different from that of massive particles in that it is always equal to the energy divided by the speed of light. In contrast, the momentum of massive particles can vary depending on their mass and velocity.

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